I've been hearing a lot of people mention how Stack Overflow is unique and innovative. I would like to know what features/capabilities of this site are innovative and unique that you won't find elsewhere.

From what I see, the sum of the parts might be innovative for this sort of offering, but the individual components can be found elsewhere....

What's so innovative about SO is that it combines all these features in a way that actually accomplishes the goal of getting answers to programming questions.

Jeff and Joel didn't set out to create a programming forum - there were already plenty of them. They set out to solve the problems that plague most forums (like unhelpful or incorrect responses). While the individual features they used can be found elsewhere, the combination of those features has enabled a truly unique site.

I agree with most other answers about the right mixture, a good community and founders with integrity, but to me, there is one more important point: The masterstroke of a user interface. You hardly notice it, and it manages to build a flow that makes writing a joy. It provides extremely little obstacles and doesn't remind you of itself on every click. It's like an old english butler always staying in the background, but always there when you need him.

Much of this is surely down to the markdown editor, but it's many other aspects of the UI as well and how things are handled. Probably the result of many hundreds of individual decisions.

I like the UI so much I actually look forward to writing text in it, and I notice how many other forums / sites / wikis / ticketing systems lack comparable smoothness.

The only exception to the rule is the god-awful human test that always pops up when I edit my answers. Why oh why! I really am for disabling that at least for higher-rep users.

Although sharing similarities with Digg's model, I really love the peer-review system (up/down votes) put in place to help sort out good answers from great answers. These are especially helpful when I come across a topic that I know little about.

As others have said, there's probably nothing truly original to SO. What makes this place so great is how preexisting things are brought together in such a way that it revolutionizes the way programmers communicate and educated one another.

Reminds me of the buzz over ajax in the last few years. Javascript, not so new. XML, not so new. Asynchronous requests, not so new. Put them all together, wha! GMail!?

The focus on the essence of questions and answers and not the inidviduals
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the strong group of users (especially the pioneers/moderators) who are clear on what they want to achieve without giving in easily to 'temptations' while practising flexibility occsionally.

No feature could truly be unique to one site or the other unless company patents some feature. (which is not easy) What I'm saying is that if a new feature is added and is really cool, then it is usually quickly copied by other sites. Not that there is anything wrong with that.

The one feature I like the most on Stackoverflow is the upvoting/downvoting along with the reputations attached with every user. The reputation system is kind of innovativeness which keeps you addicted to SO.

A good community of users who are willing to help each other by not only answering the questions but also by suggesting alternative methods to achieve the same results too.

One thing that is (at least was) unique to Stack Overflow is how it handles permissions. Lots of sites have karma and voting, but I don't know anywhere else that used it to unlock abilities like this before Stack Overflow came along. Such a site may have existed (the internet is a big place), but it wasn't as well known.